1. Field of the Invention
The present invention relates to prismatic intraocular lenses for restoring vision in patients with macular degeneration. More particularly, the present invention relates to prismatic intraocular lenses having aspheric surface geometry and related methods of their manufacture.
2. Description of the Related Art
As shown in FIG. 1, a normal eye 10 includes a cornea 12, an aqueous solution called the aqueous humor 14 behind cornea 12, an iris 16, a natural lens 18, ciliary sulcus 20, retina 22, macula 24 at the center of the retina, and fovea 26 at the center of macula 24. The cornea 12 and lens 18 cause an image 30 to form at fovea 26. Fovea 26 is a circular zone approximately 0.2-0.5 mm.sup.2 in area. The image 30 formed at fovea 26 corresponds to a locus of fixation for providing acute vision. This locus of fixation helps to coordinate voluntary and involuntary head and eye movements required for daily activities, such as reading, driving, and dressing. Peripheral images are located around this locus of fixation.
A common cause of blindness in adults is macular degeneration. This retinal disease involves damage to the fovea so that the fovea is unable to process images. The damage spreads over time into the macula and beyond, causing a blind spot at the center of a patient's visual field. The patient is thus unable to read, drive, or perform other tasks that require the brain to reference the locus of fixation.
In most patients, even in those with advanced macular degeneration, the macula is not completely damaged, but retains healthy areas. However, the loss of the locus of fixation caused by the central blind zone leads to severe visual impairment and often to legal blindness, defined as visual acuity of 20/200 or less. The number of patients diagnosed with such severe visual impairment in the United States alone exceeds 2 million.
Intraocular lens implants have been devised to replace the natural lens of the eye and restore sight to damaged or diseased eyes. The intraocular implant directs image forming light rays to a healthy portion of the macula. One such implant is an intraocular lens which forms part of a telescopic optic. Such an implant includes a convex lens forming the eye piece of a Galelian optic. The objective lens of the telescope is provided by another plus lens which may be provided in the form of a contact lens or a spectacle lens.
Compound intraocular lenses that combine different optical elements also have been proposed. In such proposals, a diffractive/refractive lens implant includes a diffractive lens profile covering about half the effective lens area. Such a configuration allows about half of the incident light from distant objects and half of the incident light from near objects to enter the eye. Such a compound optic provides an ability to form on the retina a focused image of both distant objects and near objects.
Although both images are formed on the fovea, the brightness of the image in each case is reduced by about 50%, or the ratio of the light intensity assigned to each image. In certain cases, such a lens can be used to treat macular degeneration by providing sufficient image magnification so as to project the image over a retinal area more than that damaged by macular degeneration. Such an approach, however, does not shift the image to healthy portions of the retina.
Similar multifocal intraocular lenses incorporating two refractive zones also have been disclosed. For example, the use of a pair of bifocal intraocular lenses has been disclosed in which each of the pair of bifocal intraocular lenses incorporates a refractive element and a diffractive element. One of the lenses provides greater image intensity for the image of near objects, while the other lens provides greater image intensity for the image of distant objects. This approach has the advantage that the incident light can be apportioned or split between the two images in a continuous manner between the two lenses. The disadvantage is that the image is processed by two optical elements, each of which introduces its own aberrations and loss of image contrast so that the performance of the compound lens can be worse than either a diffractive or refractive lens.
Intraocular lenses incorporating a single refractive element also have been devised to shift the image from a damaged portion of the retina to a healthy area. One such example is a prismatic intraocular lens that includes a convex spherical lens portion for focusing light rays and a prism posterior to the convex lens for deflecting light away from the diseased center of the retina to a functional point. This amount of deflection provided by the prism depends upon the prism vector, i.e. the prism wedge angle and the orientation of the prism with respect to the nasal-temporal axis of the retina.
Such prismatic intraocular lenses restore the central field vision to a patient. Prior prismatic intraocular implants, however, do not provide any means for correcting optical aberrations such as astigmatism, coma, and spherical aberration introduced by the prismatic portion. Consequently, the quality of the image delivered by prior prismatic intraocular lenses is poor, and may not provide significant benefit to patients implanted with such devices. Thus, there is a continuing need to improve the image quality provided by such lenses.